PROGRAMMAZIONE I A.A. 2017/2018

Transcription

1 PROGRAMMAZIONE I A.A. 2017/2018

2 FUNCTIONS

3 INTRODUCTION AND MAIN All the instructions of a C program are contained in functions. üc is a procedural language üeach function performs a certain task A special function name is main( ): the function with this name is the first one to run when the program starts. The first command executed in your program is the first command in main (top to bottom) All other functions are subroutines of the main( ) function can have any names you wish. Every function is defined exactly once. A program can declare and call a function as many times as necessary.

4 FUNCTION DEFINITION The definition of a function consists of a function head (or the declarator), and a function block. The function head specifies the name of the function, the type of its return value, and the types and names of its parameters, if any. The statements in the function block specify what the function does. type function_name (type parameter_name1, type parameter_name2, ) { //statements;

5 FUNCTION RETURN Always write the return-value type üit is int if not specified

6 FUNCTION DECLARATION By declaring a function before using it, you inform the compiler of its type: in other words, a declaration describes a function s interface A declaration defines the type of a function üthe number and types of its parameters üthe type of what returned The identifiers of the parameters in a function declaration are optional. If you include the names, their scope ends with the prototype itself type function_name (type parameter_name1, type parameter_name2, ); Both valid declarations type function_name (type, type, );

7 IDENTIFIER SCOPE 2 Block scope: identifiers declared within a block have block scope. You can use such an identifier only from its declaration to the end of the smallest block containing that declaration. The smallest containing block is often, but not necessarily, the body of a function definition. In C11, declarations do not have to be placed before all statements in a function block. The parameter names in the head of a function definition also have block scope, and are valid within the corresponding function block. Function prototype scope: The parameter names in a function prototype have function prototype scope. Because these parameter names are not significant outside the prototype itself, they are meaningful only as comments, and can also be omitted.

8 IDENTIFIER SCOPE 3 üfunction scope: the scope of a label is always the function block in which the label occurs, even if it is placed within nested blocks. In other words, you can use a goto statement to jump to a label from any point within the same function that contains the label. (Jumping into nested blocks is not a good idea, though). The scope of an identifier generally begins after its declaration. However, the type names, or tags, of structure, union, and enumeration types and the names of enumeration constants are an exception to this rule: their scope begins immediately after their appearance in the declaration, so that they can be referenced again in the declaration itself.

9 EXAMPLE 1 int p; int fun1 (int q); int fun2 (int r) { int s = r + 3; while( r!= s) r--; File scope Function prototype scope Block scope p, fun1, fun2 q r,s

10 FUNCTION PARAMETERS The parameters of a function are ordinary local variables. The program creates them, and initializes them with the values of the corresponding arguments, when a function call occurs. Their scope is the function block (block scope). long double factorial( unsigned int n ) { long double f = 1; while ( n > 1 ) f *= n--; return f;

11 DECLARATION AND DEFINITION A function definition is also a function declaration int fun2 (int); int main() { int a= 5; a= fun2(a); int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r; = int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r; int main() { int a= 5; //commands a= fun2(a);

12 SO Before calling (using) a function you need to declare it üyou can declare all the functions at the top of the file, and define them in any order below in the same file üotherwise you can avoid declaring function at the top, but you need to define them before they the are called A function needs also to be defined, otherwise you do not know hat to do when it is called In the same file, you cannot have two definitions of functions with the same header and identifier (name)

13 EXAMPLE int fun1 (int); int fun2 (int); int main() { int a= 5; a= fun2(a); int fun1(int q) { return (fun2(q)+1); int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r; int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r; int main() { int a= 5; a= fun2(a); int fun1(int q) { return (fun2(q)+1);

14 EXAMPLE int fun1 (int); int main() { int a= 5; a= fun2(a); int fun1 (int q) { return (fun2(q)+1); int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r; int fun1 (int); int fun2 (int); int main() { int a= 5; a= fun2(a); int fun1 (int q) { return (fun2(q)+1); int fun2 (int r) { int s = r + 5; while( r!= s) r--; return r;

15 RETURN The return statement ends execution of the current function, and jumps back to where the function was called : return [expression]; expression is evaluated and the result is given to the caller as the value of the function call. This return value is converted to the function s return type, if necessary. A function can contain any number of return statements Return is an unconditional jump statement (as goto)

16 EXAMPLE // Return the smaller of two integer arguments. int min( int a, int b ) { if ( a < b ) return a; else return b; int main() { int a= 5; int b= 6; int c= min(a,b); printf( %d, c); 5

17 RETURN (2) A return statement with no expression can only be used in a function of type void. In fact, such functions do not need to have a return statement at all. If no return statement is encountered in a function, the program flow returns to the caller when the end of the function block is reached.

18 MAIN

19 MAIN() FUNCTION You can define the main( ) function in one of the following two forms: int main( void ) { /*... */ A function with no parameters, returning int int main( int argc, char *argv[] ) { /*... */ A function with two parameters whose types are int and char **, returning int

20 PARAMETERS OF MAIN() The parameters argc and argv (which you may give other names if you wish) represent your program s command-line arguments. This is how they work: argc (short for argument count ) is either 0 or the number of string tokens in the command line that started the program. The name of the program itself is included in this count. argv (short for arguments vector ) is an array of pointers to char that point to the individual string tokens received on the command line: üthe number of elements in this array is one more than the value of argc; the last element, argv[argc], is always a null pointer. üif argc is greater than 0, then the first string, argv[0], contains the name by which the program was invoked. If the execution environment does not supply the program name, the string is empty. üif argc is greater than 1, then the strings argv[1] through argv[argc - 1] contain the program s command line arguments.

21 A CLASSICAL EXAMPLE #include <stdio.h> int main( int argc, char *argv[] ) { if ( argc == 0 ) puts( "No command line available." ); else { printf( "The program now running: %s\n", argv[0] ); // Print the name of the // program. if ( argc == 1 ) puts( "No arguments received on the command line." ); else { puts( "The command line arguments:" ); for ( int i = 1; i < argc; ++i ) puts( argv[i] ); // Print each argument on // a separate line.

22 OUTPUT FOR THE EXAMPLE Suppose we run the program on a Unix system by entering the following command line: ü$./args one two "and three" The output is then as follows: The program now running:./args The command line arguments: one two and three

23 RETURN OF MAIN The value returned by main is a value that is passed The return value for main should indicate how the program exited. Normal exit is generally represented by a 0 return value from main. Abnormal termination is usually signalled by a non-zero return The OS returns this value to the process which called the executed program (e.g., the shell)

24 HOW FUNCTIONS ARE EXECUTED

25 RETURNING FROM A FUNCTION If the program reaches a return statement or the closing brace of the function block, execution of the function ends, and the program jumps back to the calling function. If the program falls off the end of the function by reaching the closing brace, the value returned to the caller is undefined. For this reason, you must use a return statement to stop any function that does not have the type void. The value of the return expression is returned to the calling function.

26 CALL A FUNCTION The instruction to execute a function the function call consists of the function s name and the operator ( ) For example, the following statement calls the function maximum( ) to compute the maximum of two numbers ümaximum( a, b); The program first allocates storage space for the parameters, then copies the argument values to the corresponding locations. Then the program jumps to the beginning of the function, and execution of the function begins with first variable definition or statement in the function block.

27 WRONG i and j are passed by value! p 3 #include <stdio.h> q 5 int sum(int p, int q) { int r= 0; r= p + q; return r; r 08 int main( void ) { int i =3, j =5, int s= 0; s= sum(i, j); printf( %d\n, s); return 0; i j s 08 Memory

28 VARIABLE ALLOCATION Memory allocation means that part of the memory is reserved, for instance for a variable Automatic variable is a local variable which is allocated and deallocated automatically when program flow enters and leaves the variable's scope By memory deallocation we mean that part of the memory is no longer reserved, for that variable

29 OVERLOADING OF FUNCTIONS

30 NOT POSSIBLE IN C! Function overloading is a feature of a programming language that allows one to have many functions with same name but with different signatures (parameters type and return type) üthis feature is present in most of the Object Oriented Languages such as C++ and Java übut C doesn t support this feature To overload a function, in C++ you an define it several times, each uniquely identified by the type of the arguments it accepts; return type is not considered

31 EXAMPLE #include<stdio.h> int min(int a, int b) { return ( x < y? x : y); float min(float a, float b) { return ( x < y? x : y); int main( ) { int a= min(4, 1); float b= min(4.56f, 1.23F); MacBook-Francesco:ProgrammI francescosantini$ gcc esempio.c esempio.c:8:7: error: conflicting types for 'min' float min(float a, float b) { ^ MacBook-Francesco:ProgrammI francescosantini$ g++ esempio.c